1. Field of the Invention
The present invention relates to an impedance transformation circuit and a wireless communication apparatus preferably for use in an antenna device or other suitable device.
2. Description of the Related Art
Frequency bands used in cellular phones in recent years are very broad bands, and for example, a penta-band-supported cellular terminal is required to support both of a Low Band (for example, 824 MHz to 960 MHz) and a High Band (for example, 1710 MHz to 2170 MHz). For this reason, in order to allow a single antenna to support both of the Low Band and the High Band, the antenna is assigned with different operation modes according to frequency ranges. In normal circumstances, the antenna is designed to support the Low Band in a fundamental wave mode, and is designed to support the High Band in a higher harmonic wave mode. Then, the input impedance of the antenna is different according to the modes (resonance point). Antennas for cellular phone terminals have, for example, impedance of about 8Ω in the Low Band and impedance of about 15Ω in the High Band.
A matching circuit based on a transformer is used to attain matching between a feeder circuit and an antenna having different input impedance according to frequency bands as described above. In this case, when a transformer ratio is constant, matching is attained at any one of the frequency ranges and then matching cannot be attained at the other of the frequency ranges. Therefore, a matching circuit having different impedance transformation ratios according to frequency bands becomes necessary. For example, Japanese Patent Unexamined Publication No. 2012-191596 describes an impedance matching circuit having frequency characteristics (frequency dependency) by giving a reactance element to an impedance transformation circuit based on a transformer.
By the way, in a transformer having frequency characteristics which attains impedance matching in a desired range, the impedance at the antenna port side can be said to be equivalent to antenna impedance for each frequency. When the antenna port-side impedance of the transformer is matched to the impedance of the antenna in the Low Band and the High Band, and a coupling coefficient that could be attained with the actual structure is considered, combinations of an inductance L1 of a primary coil and an inductance L2 of a secondary coil used for coupling are limited to two combinations. Then, the values of L1 and L2 are very small, e.g., several nH, and a structure becomes difficult to obtain a high coupling coefficient because of the following factors.
When coils of about 2 nH are coupled, a sufficient number of turns of each coil cannot be secured (the magnetic flux is not concentrated).
The inductances at the input and the output of the transformer become large in an overall proportion and the effective value of the coupling coefficient becomes small.
In order to obtain a predetermined (large) coupling coefficient with a small coil, it is effective to configure the shapes of the primary coil and the secondary coil to be the same shape or substantially the same shape (i.e., a shape close to congruent) and arrange the primary coil and the secondary coil so as to overlap each other.
However, when the primary coil and the secondary coil have the same shape, it is very difficult to obtain a desired inductance value for each of the primary coil and the secondary coil.